Figure 4. Typical chlorophyll fluorescence emission trace for a light-adapted leaf made with a PAM fluorometer. Interpretation: Pulse 1 = measuring beam on; F0 = non-electron transport-stimulating fluorescence emission ‘background’ eliciting the minimum fluorescence value; Fm is the maximum fluorescence; Fv is variable fluorescence Δ Fm – F0; Pulse 2= high-intensity actinic light on; F′ (F-prime), steady state fluorescence; Fm′ is the maximum steady-state fluorescence; Fq′ is the difference between Fm′ and F′; FR=pulse of far-red light to stimulate PSI; F0′ is the minimum fluorescence value after switching on FR; Fv′ is variable fluorescence in light Δ Fm′ – F0′. On the right are emission parameters and their formulas: ΦII = quantum efficiency of PSII electron transport in light; qP: photochemical quenching coefficient or ‘efficiency’ factor; qN: non-photochemical quenching coefficient or ‘dissipation’ factor; NPQ, non-photochemical quenching or the dissipation of light energy through non-photosynthetic mechanisms; qL, fraction of open PSII reaction centers; LEF, rate of electron flow in PSII. Source: adapted and re-drawn from Murchie and Lawson, 2013.